1. Field of the Invention:
The present invention relates to a device for calibrating a radiation detector system that is used for measuring the radionuclide intake of those exposed to radioactive materials. In particular, the present invention relates to a device that simulates a human chest and lungs with a modicum of internal radiation for use in calibrating radiation detectors.
2. Discussion of Background:
Present-day occupational safety standards mandate that personnel exposure to ionizing radiation be as low as reasonably achievable. The recommended maximum permissible total body close for occupational exposure from all sources during any one-year period is 5 rem, independent of exposure for medical reasons or exposure to natural background radiation. Additional standards govern the maximum permissible intake of individual radionuclides. Maximum permissible intakes depend on the energy, activity and half-life of the particular radionuclide, the chemical properties of the radionuclide, the organs that accumulate the radionuclide (lungs, thyroid, kidneys, etc.), and the mode of intake (oral, by inhalation, or by a wound). The Annual Limit on Intake (ALI) by inhalation is lowest for Class Y materials (radionuclides with half-lives measured in years), and higher for Class W and Class D materials (half-lives measured in weeks and days, respectively). By way of example, the ALIs for .sup.234 Th and .sup.235 U, both Class W materials, are 190 .mu.Ci (7.0 MBq) and 810 nCi (30,000 Bq), respectively. Data on ALIs are found in the International Commission on Radiological Protection (ICRP) Publication 30, "Limits for Intakes of Radionuclides by Workers"; these data are incorporated herein by reference.
Current regulations mandate routine exposure monitoring and regular bioassays for all personnel who may be exposed to transuranic materials. Typically, workers carry personal radiation dosimeters such as thermoluminescent dosimeters (TLDs) to record their total external exposure during a given time period, and work areas are furnished with detectors and alarms to monitor ambient radiation levels and warn of potentially-hazardous conditions. Work time in high-radiation areas is limited, sometimes to no more than a few minutes, to limit total exposure to permissible levels. Protective clothing and equipment may be required for persons working with radionuclides or in high-radiation areas.
In addition to these precautions, lung counts are carried out at intervals for all potentially-exposed personnel to monitor the uptake of radionuclides by the lungs ( called the "lung burden"). In most cases, the total exposure and lung burden are very low, so an annual lung count, in conjunction with in-vitro (urine) sampling, serves primarily to verify that field monitors and personal dosimeters are working properly. Following accidental exposure to unusually high levels of radioactive materials, such as might occur during a spill or accidental release of nuclear materials, lung counts help quantify the overall internal exposure and the types and amounts of radionuclides that are present in the body.
Lung counts involve counting rates that are lower than normal background radiation levels, sometimes by a factor of ten or more depending on the location of the test site. Background counts vary widely depending on factors such as the elevation (higher background counts are found at higher elevations), the type of building (higher counts are found in concrete buildings than in wood buildings), and the amount of radon in the area. Lung counts are usually done in shielded rooms in order to reduce the contribution of spurious background radiation to the subject count. Background radiation levels can be reduced to very low levels by shielding the walls, ceiling and floor of a room with stainless steel or other highly attenuating material.
To perform a lung count, two arrays of radiation detectors are positioned close to the chest of the subject being counted, one array on each side, and the detectors are operated for a period of 30 minutes or more. The energy (keV) and activity (nCi; Bq) of any radionuclides in the subject's lungs are computed using the measured count, the radiological attenuation properties of human tissue and the response characteristics of the detectors. These data are used to evaluate the subject's overall exposure status, and the need (if any) for medical treatment and precautions regarding future occupational exposure.
Referring now to FIG. 1, there is shown a typical apparatus 10 for performing lung counts on human subjects. Apparatus 10 includes a detector system 12 as well as counters, data analysis and recording equipment, visual displays and so forth (not shown). Detector system 12 has two detector arrays attached to a support 14, a first array 16 with a plurality of detectors 18a, 18b, 18c, and a second array 20 with a plurality of detectors 22a, 22b, 22c. Detectors 18, 22 may be in the form of so-called "organ pipe" or "stove pipe" detectors such as are known in the art, approximately 4" (about 10 cm) in diameter and 27" (about 69 cm) long. A subject 24 reclines in a chair 26 for the lung count, with one detector array placed on each side of his or her chest. Chair 26 is attached to a frame 28 that allows the height and lateral position of chair 26 to be adjusted.
For optimum results, detectors 18a, 18b, 18c are placed with their forward ends 30a, 30b, 30c as close as possible to one side of the chest of subject 24 but not so close as to interfere with the subject's breathing, preferably just touching the chest. Forward ends 32a, 32b, 32c of detectors 22a, 22b, 22c are placed on the other side of the subject's chest.
To accommodate subjects with differing physiques, support 14 is slidable on a rail 34, and detector system 12 is rotatable about an arm 36 attached to support 14. The positions of individual detectors 18 of array 16, and detectors 22 of array 20, are fixed with respect to each other. However, arrays 16, 20 may be movable in tandem in a direction parallel to the longitudinal axis of detector system 12, indicated by an axis "z" in FIG. 1. Axis z is roughly parallel to the longitudinal axes of individual detectors 18, 22. Arrays 16 and 20 may also be positioned independently of each other along axis "y", perpendicular to axis z. It will be understood that the arrays 16, 20 (and detectors 18, 22) may have other degrees of freedom than those described above. For example, arrays 16, 20 may be movable with respect to each other along axis z, arrays 16, 20 may be separately rotatable about arm 36 or axis y, and individual detectors 18, 22 may be movable with respect to each other along axes "y" and "z".
To accommodate the shape of the chest, forward ends 30 of detectors 18, and forward ends 32 of detectors 22, respectively, are non-coterminal. As used herein, the term "non-coterminal" means that forward ends 30 do not terminate at the same point and do lie in the same plane, because each of ends 30 is aligned at a different angle with respect to axis z. In addition, arrays 16 and 20, when in position, each have at least one detector with a forward end that is non-coplanar with at least one detector of the other array.
Detectors 18, 22 are tested and calibrated on a regular basis to ensure the reliability of the counts. "Full calibration" is preferably done once per year. A model of the human body (dummy; mannekin; phantom) is used, preferably a model having the approximate size, shape, radiological density and effective atomic number of human tissue, with appropriately-positioned radionuclide-containing "lungs." Arrays 16, 20 are positioned on the "chest," and counts are taken with varying thickesses of overlay material. The "lungs" may contain a single radionuclide or several radionuclides with different activities. The results are used to determine the operational parameters of the system, and to evaluate the overall variability of counts taken using apparatus 10, including the effect of different activities on the measured count and the variability of detector positioning on the human body.
Full calibration is tedious and time-consuming, requiring several days to complete. Therefore, a faster "daily calibration" or "check source" is done more frequently, usually before each day's scheduled tests. However, daily calibration may be done on a different schedule, such as on alternate days or twice per day, depending on the degree of assurance desired. Daily calibration involves placing detector arrays 16, 20 on a model, recording the output of detectors 18, 22 for a suitable time interval, and computing the energy and activity of the source from the recorded data.
For a valid comparison between daily calibration counts, the amount of radiation reaching detectors 18, 22 from a known source must be reproducible. It is well known that the recorded count at any radiation detector depends on the overall geometry, including the distance from a source of ionizing radiation, the angle of incidence of the radiation reaching the detector, the relative positions of other sources (if present), and the radiological density of intervening materials. All of these factors must be reproducible. Therefore, the same model must be used for each daily calibration, preferably a model that contains a radionuclide with a long half-life and a stable activity that is comparable to the activity measured from typical human subjects. The positions of detector arrays 16, 20 (and detectors 18, 22) must be reproducible with respect to the model and with respect to each other. Without such reproducibility, there can be no assurance that apparatus 10 is functioning within performance guidelines.
The full-size models used for full calibration are relatively accurate representations of the human body, with internal radionuclide-containing "lungs" (and other organs if desired). These devices are large, heavy and cumbersome, therefore inconvenient to use for routine testing. In addition, the devices are costly and not sufficiently durable for day-to-day use. Even if such devices were used for daily calibration, they are not adapted for precisely and reproducibly positioning detector arrays 16, 20 with respect to the "lungs" and with respect to each other. The more degrees of freedom for the movement of chair 26 and arrays 16, 20, the more difficult it is to reproduce their relative positions even approximately.
Presently-available daily calibration models include a flat "source plate" that carries a radiation source, and an overlying flat "shield plate" of PLEXIGLAS.TM. or some other material that mimics the radiological characteristics of tissue. Such models are useful for testing a single detector, but they are not suitable for testing a plurality of movable detectors or movable arrays. As noted above, detectors 18, 22 are designed for placement against a human chest, thus, detectors 18 (and detectors 22) are non-coterminal, and forward ends 30, 32 of detectors 18, 22 are non-coplanar. It is impossible to position detectors 18, 22 so that all forward ends 30, 32, respectively, engage a single flat surface. More importantly, because of the very low levels of activity to be measured (as low as 10 nCi), detectors 18, 22 are extremely sensitive and even small variations in the relative positions of arrays 16, 20 can lead to unacceptable variations in geometry. It is impossible to precisely and reproducibly position arrays 16, 20 so that detectors 18, 22 are in the same relative positions with respect to this type of model and with respect to each other. Therefore, it is difficult to compare daily calibration counts to determine whether apparatus 10 is operating properly. Individual sources and plates might be used for testing the individual detectors 18, 22. However, the relative positions of arrays 16, 20 must still be reproduced and the detectors tested together to determine whether or not the overall system is working properly.
Models or phantoms of the human body are frequently used in other situations. For example, phantoms are used to test automobiles and automobile accessories such as safety belts and air bags (Haurat, et al., U.S. Pat. No. 3,890,723; Melzian, U.S. Pat. No. 3,648,389). Phantoms are used in radiotherapy to simulate the human body in order to determine radiation absorption and calculate a therapeutic dose for a subject (Alderson, U.S. Pat. No. 3,310,855). The Alderson device is formed from a plastic material or FIBERGLAS.TM. in approximately the size and shape of the human torso, and contains several internal cavities for placing detectors to measure incident radiation. However, there is no presently-available device that contains a source that simulates radionuclide-containing organs, against which one or more arrays of detectors can be reproducibly positioned for the purpose of measuring the emitted radiation to assure the reliability of the detectors and associated equipment. A suitable device for daily use would be relatively light, simple and easy to set up and move, inexpensive, and durable.